A few weeks ago I was attending the Missouri State F-Class Match, this was a two-day event during the summer and temperatures were hot one day and hotter the next. I shot next to a gentleman that was relatively new to the sport. He was shooting a basically factory rifle and was enjoying himself with the exception that his scores were not as good as he hoped they would be and he was experiencing pressure issues with his ammunition. I noticed that he was having to force the bolt open on a couple of rounds. During a break, I visited with him and offered a couple of suggestions which helped his situation somewhat and he was able to finish the match without major issues.

He was shooting factory ammunition, which is normally loaded to upper levels of allowable pressures. While this ammunition showed no problems during “normal” testing, it was definitely showing issues during a 20 round string of fire in the temperatures we were competing in. My first suggestion was that he keep his ammunition out of the direct sun and shade it as much as possible. My second suggestion was to not close the bolt on a cartridge until he was ready to fire. He had his ammo in the direct sunlight and was chambering a round while waiting on the target to be pulled and scored which can take from a few seconds to almost a minute sometimes. This time frame allowed the bullet and powder to absorb chamber temperature and build pressure/velocity above normal conditions. Making my recommended changes lowered the pressures enough for the rifle and cartridge to function normally.

After thinking about this situation, I decided to perform a test in the Sierra Bullets underground range to see what temperature changes will do to a rifle/cartridge combination. I acquired thirty consecutive 30 caliber 175 grain MatchKing bullets #2275 right off one of our bullet assembly presses and loaded them into 308 Winchester ammunition. I utilized an unnamed powder manufacturer’s product that is appropriate for the 308 Winchester cartridge. This load is not at the maximum for this cartridge, but it gives consistent velocities and accuracy for testing.

I took ten of the cartridges and placed them in a freezer to condition.

I set ten of them on my loading bench, and since it was cool and cloudy the day I performed this test I utilized a floodlight and stand to simulate ammunition being heated in the sun.

I kept track of the temperatures of the three ammunition samples with a laser non-touch thermometer.

The rifle was fired at room temperature (70 degrees) with all three sets of ammunition.

I fired this test at 200 yards out of a return-to-battery machine rest. The aiming point was a leveled line drawn on a sheet of paper. I fired one group with the scope aimed at the line and then moved the aiming point across the paper from left to right for the subsequent groups.

Please notice that the velocity increased as the temperature of the ammunition did.

The ammunition from the freezer shot at 2451 fps.

The room temperature ammunition shot at 2500 fps.

The heated ammunition shot at 2596 fps.

The tune window of the particular rifle is fairly wide as is shown by the accuracy of the three pressure/velocity levels and good accuracy was achieved across the board. However, notice the point of impact shift with the third group? There is enough shift at 200 yards to cause a miss if you were shooting a target or animal at longer ranges. While the pressure and velocities changed this load was far enough from maximum that perceived over pressure issues such as flattened primer, ejector marks on the case head, or sticky extraction did not appear. If you load to maximum and then subject your ammunition to this test your results will probably be magnified in comparison.

This test showed that pressures, velocities, and point-of-impact can be affected by temperatures of your ammunition at the time of firing. It’s really not a bad idea to test in the conditions that you plan on utilizing the ammo/firearm in if at all possible. It wouldn’t be a bad idea to also test to see what condition changes do to your particular gun and ammunition combination so that you can make allowances as needed. Any personal testing along these lines should be done with caution as some powder and cartridge combination could become unsafe with relatively small changes in conditions.

65 Responses to Does Temperature Affect Point of Impact?

All powders are sensitive to temps, just some are wildly so and can become dangerous if the charge is already at or past maximum. Those powders people claim are not sensitive, are just less sensitive.
Yes, plenty of people shoot past maximum loads, just think it is going to be okay, at least until it is not any more. 🙂

Great article with empirical data to back it up.
When we shot a lot of Cowboy Action, one time I noticed some issues with the ammo. Not a readily available loading, used Green Dot in .38/.357 brass behind 125 and 158 grain hard cast lead bullets. Had an unusually hot day during a match and started having some primer issues. Nothing serious, but getting some drag on the recoil shield of the revolvers. Put all the ammo away in the shade of the gun cart, all the issues went away. 🙂 Realized the ammo, sitting in the sun, coupled with the high air temps, was causing it to go about 150 or so. In the morning everything was fine, but as the day went on, the ammo heated up, coupled with the firearms heating up too, caused some problems.
This is a non-conventional loading, but because of Green Dots lessened apparent recoil, compared to other powders, it worked extremely well. To the point of others asking me what we were shooting often. Almost no muzzle flip, yet decent knock down of the steel targets most used. One friend whom shot 32-20 because of the low recoil, had to borrow one of our rifles once, was amazed at the lack of recoil and much better knock down ability. At the end of the match, she told her husband to sell the 32-20s and get her some .38/.357s. After having him talk to me about my loadings. 🙂
Reloading is wonderful!!! Really appreciate all the great information you and your company takes the time to share with us!!

Great article! I wish my rifles shot this well (of course, it could be the shooter…). The photo of the target indicates the hot rounds had a higher average velocity but a LOWER point of impact. I would have thought the point of impact would have gone up as the cartridge temperature and velocity increased. Am I missing something here?

What you are seeing is barrel harmonics. Think of the barrel as a whippy pipe, which is what they are. As the barrel vibrates when the charge goes off, the muzzle moves around, if the timing is such that the bullet exits the barrel when it is in a “lower” position, the round will strike lower, even though the velocity is higher.

if I’m not mistaken the subject is how ambient temperature impacts POI and not barrel harmonics but I could be mistaken. Harmonics is an entirely different subject or so I thought..

I’m sure my barrels vibrate a bit, but it’s of no concern to me. The smallest kill zone I deal with is 14″ unless it’s a squirrel and 99% of the squirrels I shoot are with a 410 shot shell anyway. Never been a paper puncher though I have ‘punched’ a bit with Brian Litz and John Pierce for fun

“I would have thought the point of impact would have gone up as the cartridge temperature and velocity increased. Am I missing something here?”
You asked the question, the answer is yes.
The point was how temps affect the speed, pressures and conversely bullet timing, which is influenced by harmonics of the barrel.

Mr. Todd would need to verify, but if he is using a universal receiver, I do not think they actually recoil as those I have seen in the past were immobilized in the fixtures being used. Which if this is the case here, would eliminate the effect of recoil, I would think.
I have seen a number of recording of pistols being fired and the bullet leaves the barrel before anything actually starts to move, at least to appearances on the recorded image.

Great article! Increasing velocity with increasing temperature certainly is expected; as firstriverbend says, how much velocity changes depends on how sensitive the powder is to temperature changes. However, if I understand the target photo correctly, the hottest rounds had a LOWER point of impact. I would have thought that point of impact would have risen as the velocity increased. Any additional thoughts you have on this result would be greatly appreciated.

I am just the average reloader I used to shoot metallic silhouettes back in the late 70s and early 80s we found out back then just what your results show by trial and error a lot of guys would keep there shells in a cooler even on the line as they shot but what I tried was to develop the loads for accuracy when it was the hottest days of summer what I found out was that as the Temps dropped in the pistols I was using the accuracy was pretty much the same even at 200 meters after that I started doing the same thing for my hunting rifles with the same results, it would be interesting to see if you would get the same results under your more technical way of testing reversing the process.

I have no doubt whatsoever that POI would change with temperature variations.
My only question is in regards to the downward shift in POI with the higher velocity. Any idea on why this is? I would have “ass-umed” an upward move as opposed to a downward one.

Think of a barrel as a water hose, it more like one than people think. As the bullet travels down the tube, the barrel moves around. This is just like a water hose that is turned on but held back a foot or so from the end. What happens then? Well the hose shoots a stream of water out some going down, some going up, some going right and left. Barrels do the same thing.
When you develop a load, you are finding the point of timing the bullet exit to the barrel’s harmonics, so the bullet leaves the barrel when the end of the barrel is in the same position each time. When you have the same speed, pressure, and a whole host of other things exactly the same each time, the bullet will start on the same path each time.
Then the wind, takes its turn at moving things around.
The upshot, is if the bullets are exciting the barrel at a different velocity, the bullet is getting to the end of the barrel at a different time. This means the barrel is going to be pointed to a different point, all other things being equal. https://www.youtube.com/watch?v=FaW_Hs0B79c
This is a pretty good video of an air rifle barrel, shows how jello like barrels are, does not matter if it is and air rifle or a 16 inch gun on a battle ship, they all do the same thing, just in varying degrees. I picked this video, because of the clarity.
Watch what the rear sight does. 🙂

Nate, Yes, bull barrels do affect the movement, but does not eliminate it.
The thicker the walls of the barrel are, the smaller the motion of vibration/harmonics are. It does not make it go away, just generally lessened the amount of total movement of the end of the barrel.
All barrels need to be compromises of the various properties, i.e. do you really want a rifle for hunting that weights 25 pounds, even though it is easier to hit the target at say 1,000 yards? Or do you want a rifle that is only 4 pounds with a barrel pencil thin, but after two shots, the point of aim is 6 inches from where you started and it will only shoot one load accurately?

Thank you again for a great article. my dad taught me 50 years ago to keep all my ammo out of the direct sunlight while on the range. I shoot competitive rimfire silhouette and am amazed at the shooters that leave their ammo in the shooting stands in the direct sunlight of South Mississippi. I know you guys aren’t rimfire manufactures but I sure wish someone would write the same article about 22’s!

This article is very interesting. In the past month, I have pulled the Sierra 175 HPBT, dumped the BL-C2 powder from the cases, and reloaded with Varget that from 0 degree F to 130 degree F only has a 8 FPS difference in velocity. IMR 4064 comes in second with 45 FPS spread. Lastly, for extra caution of pressure problems, I take ‘all my bullets’ with Hollow Points, and I carefully shave the tips with a fine file so all will be the exact same length of 1.239″ for perfect depth into the cases.
Thanks for the read.

I wanted to say “thanks” to Sierra Staff for not only seeing an issue with loaded rounds..but also taking the time to help a fellow shooter whom was struggling. The data provided is something some shooters have known for a long time and something newer shooters have to experience to believe. I as a shooter have used old refrigerators to simulate rifles and the ammunition in cold weather climates for hunting/shooting trips. I also use an old refurbished refrigerator with heat lamps to bring up a weapon to heated temps as well. This practice has served me well over the last 30 years.I have data searched the proposed hunt for its time and conditions, generated averages and then tuned for those conditions. The practice does not eliminate all errors..but it does try to bring down the “murphy factor”.

Very good article. Tips from someone has had his life on the line if I missed: Heat, humidity, rain, location, wind, bright\cloudy day, reliability of the loader, and me having a good\bad day all come into play. 300 or 800 yard shots, if the window of opportunity is only a square foot, you better understand and know how to correct for EACH element of a good shot. Always something to learn from old dogs…

I have found a noticeable difference in my rifles zeroes with different rifle and ammo temperatures. I also found that moving from flat, hot Florida to a much colder climate and higher elevation (3500-4000 FT and 10-20 degrees) just magnifies the problem some when going down into the flatlands to shoot. I used to have a master and high master classification so maybe I’m not too off base thinking this.
And like many others my home range was limited in distance (100-200) so ballistic problems were a critical thing to consider when I shot a match on a range I had never seen at 600-1000 yds in my discipline, high power.
It’s good to hear Sierra is still making great completion bullets and working to educated shooters more.

POI in this case has nothing to do with velocity and everything to do with barrel harmonics as stated above. This load appears to have a good powder node ( different than seating depth node )from 2450 – 2500 fps as seen by the POI being the same. ( This 50 fps difference would show up at longer range with a POI change ) But somewhere above 2500 fps the peak pressure exceeded the peak pressure of the lower velocity rounds. The bullet left the barrel at a different time during that velocities vibration sequence. It could have very easily been high of original POI.

Peak pressure occurs only after the bullet travels 2-3″ down the bore. After this the pressure drops dramatically but continues to accelerate the projectile. How fast its peak velocity is, is determined by the burn rate of the powder. This is why it is important to pick a powder with a correct burn rate for the cartridge and its barrel length.

My current 308W load with H4895, 175 Sierra and 20″ barrel produces 2540 fps at the muzzle with a measured 54K pressure. To get the same velocity increase as above would add 4K to the pressure. By the data above I am guessing the powder used is either RL15 or IMR 4064 which are excellent for the 308W but not very stable in varying temps.

A couple of years ago I was trying to thin out a prairie dog town in western Kansas when the bolt on my rifle started sticking. The longer I shot the stickier it got so I just sat back and asked myself what was different from there at that time and the time when I worked up the load I was using. The load was worked up on a covered firing line and those pesky prairie dogs were nowhere near a shade tree. There was my ammo on the bench in full sun. After I put the ammo box on the ground in the shade of the bench the sticky bolt problem went away.

Larry Ross – you are on the money with keeping your ammo shaded. Also, keep it out of the chamber until you’re ready to shoot. Never put a wet round in your chamber. Remember when the sun is behind you, your shot will be a bit high after 300 yds. There’s more, but it’s old fart time to hibernate.

The 308 is not a good rifle for this type of test if you wanting to test the difference in accuracy also. Unlike other rifles that have a larger variation between accuracy nodes. the 308 just eats anything and even your farthest deviation from an accuracy node is still pretty accurate. Some calibers may go from shooting groups in the 3′ to 2″ group or bigger depending on load. 308 dont seem to go over 1″ no matter what load I use in it. Hard to fine tune loads when everything shoots well. 🙂

With the faster bullets striking below the slower ones, I think that’s happening because the bore axis is at a lower angle than the higher ones. All bullets leaving on the muzzle axis up swing will spread groups vertically;

It’s called positive compensation. Slower bullets leaving at higher muzzle angles than slower ones will strike the same point of aim way down range for both velocities. The British proved that over a hundred years ago, Here’s some links to such stuff:

Sierra’s Infinity software easily shows the temperature effects on trajectory. Warm air is less dense than cold air. Benchresters shooting bullets barelyu fast enough to stabilize them through 300 yards will add another click or two to their measured so another tenth or so grain of powder will push the bullet out faster in cold weather. This compensates for both lower temperatures of powder and air; both of which means bullets will move slower when and after they’ve left the barrel.

Given the point of impact changes (exit timing of the harmonic node) and the TIME of FLIGHT difference due to the velocity change, one wonders about the ethical concerns of some long range hunters who profess to shoot at game animals at 1000 yds and beyond.

Paper and steel targets seldom get away if wounded, animals are a different matter. Point of impact changes due to weather/atmospherics, velocity changes and harmonic nodes become very difficult to determine. Use your Sierra software to determine the point of impact difference by adding these variables. Earn your “Stalking Merit Badge” and get a bit closer to the animal for surgical shot placement and a swift humane kill.

Totally agree with your comment Darrell!
Long ago, i largely stopped rifle hunting and shifted to mostly pistol hunting, primarily because I was shooting pretty accurate rifles and doing well shooting NRA high power matches. Delivering a kill shot outwards of 600 hundred yards stopped being sport from an accuracy point of view, but being certain it would be a humane kill was a real issue for me.
Yes, I “knew” the bullet was going to strike the animal, but because of all of the variables of the day, weather, terrain, temperature, etc… being certain of a quick one shot kill was not really certain, as it cannot be for anyone shooting true longer ranges.
My answer was to move to pistols for hunting. When you walk up to 35 yards or less, before taking the shot. I was fairly certain of clean one shot kills.
What I found was it was not impossible to approach, or have them approach me, to ranges of as close as 5-7 yards often. Most of my hunt shooting was being done between 15-25 yards, deer, elk, squirrels, etc…
I feel there has been too much emphasis placed on “how far away can I take an animal?”, not enough on how close can I approach the animal.
Thanks for a great reaffirmation of this philosophy.

Thanks for reply, The idea of “Just Get a Bullet in’ em” is becoming the mantra is some circles and the game animal suffers from such lunacy. The adrenaline rush from “getting close” is remembered far longer than: was the shot 863 or 897 yds. Woodsmanship and being a hunter are skill sets that are declining in America.

Darrell,
You are so right about the adrenaline rush!!
Of all the game I have taken over the years, really remember only two animals.
The first was my first elk at age 15, which I took with one shot at around 375 yards, according to my uncle, whose .338 Win mag I was shooting. However, the main reason I remember that one, was because it was the first time I had shot with a scoped rifle, not that common in 1965, and it was uphill. To get a better view, I snuggled up to the scope, most can figure what happened next. I heard more than felt the edge of the scope cutting into my scalp!! Did not even know what had happened, I was pretty excited by dropping that elk, until I kept having to wipe something out of my eye!!

The second one was much more mundane, thank goodness! I had spotted a 3 point buck and was able to get up to about 20-25 yards away. Took one shot with my .44 magnum Ruger pistol and it dropped like a stone! 🙂
Was concerned about penetration and had been reading Elmer Kieth a lot. My daughter asked me why it had two holes in it when I had hung it at home. It was only then that I realized the bullet had passed completely through this deer.
Lot of memories with those two animals, lot of family connections too. The first pistol hunt really stands out, as I was able to be intimate with the animal and then had much my daughter and I shared later!!
I never hunted big game with a rifle again, getting close and personal was much more interesting to me and I felt much more connected to the animals I took after that. Never had to use more than one round to bring them down either. As I was always careful about the shot.
Thanks for the jogging of some wonderful times.

As the faster bullets printed lower on paper, that may have happened because they left sooner on the muzzle axis up swing. That’s been a known cause of vertical shot placement for decades. Here’s some sites explaining why:

With the load having highest average velocity and striking lower than the slower ones, I think that happened because the bullets all left on the muzzle axis upswing. When that happens, faster ones will print lower, slower ones higher.

I have been reloading for 50+ years, and have learned that heat effects pressure, even on the so called Extreme powders. What surprised me, was the very little change of POI from 25 to 70 degrees. I have hunted my whole life, all of North America and several safaris in Africa, and have shot nothing but my hand loads . This is great information and advise for the hunter / hand loader who works up his loads in the summer.
Paul R. Kawczk

Here’s the novice question, Why did the point of impact drop when temp (and velocity) went up? Given the sled shooting, I would have figured increased velocity is less drop over the same distance, all else being equal. What am I missing here?

Which is why there are so many books written on the subject. More over ballistics is divided, in to two areas, internal – what happens inside the barrel, along with external – what happens after the projectile exits the barrel.
It is a complex and often not very intuitive study, with several variables. 🙂
I find it is a fascinating study though.
We need to all be thankful for people like the Sierra staff and their work with both, giving us a great deal of important information every day!!

I experienced cold drastically effecting accuracy once stalking deer in Scotland one winter. Took me a while to work out because when I got back to the lodge I was getting great accuracy (the gun was emptied and live rounds put in my warm coat pocket of course). A change of propellant sorted this. Most of us fully experienced in load development like a hot day for pressure testing a potential hot round. No good developing a hot load in winter then having them give you issues mid summer.

It can also be more than just the propellant, primers can be greatly effected by the cold too. When cold they can have a marked difference in ignition and burn rate, causing a big change in powder burn.

Look up “barrel harmonics”.
People make the mistake of thinking barrels are rigid tubes, nothing could be farther from the truth! Barrels move around a great deal while the bullet moves down the barrel. If the barrel is flicking in a downward motion when the bullet exits, the point of impact will move downward, even though velocity might have moved higher.
There are several good books on the subject, along with many YouTube videos of what is happening to both the barrel and receiver after the powder charge is ignited.
Read the article posted on this site about firing a “ladder” to find the sweet spot during load development. This is a graphic example of the results of barrel harmonics, along with some other variables tossed in too.
Enjoy. 🙂

Rich – The shift to a lower impact location with a higher velocity is due to the harmonics of the rifle barrel which overcame the amount of drop the bullets have at 200 yards. Often when a ladder test is shot with a fairly wide window a rifle will show this performance.